Vehicle emission control systems may be configured to store fuel vapors from fuel tank refueling and diurnal engine operations, and then purge the stored vapors during a subsequent engine operation. In an effort to meet stringent federal emissions regulations, emission control systems may need to be intermittently diagnosed for the presence of leaks that could release fuel vapors to the atmosphere. Evaporative leaks may be identified using engine-off natural vacuum (EONV) during conditions when a vehicle engine is not operating. In particular, a fuel system may be isolated at an engine-off event. The pressure in such a fuel system will increase if the tank is heated further (e.g., from hot exhaust or a hot parking surface) as liquid fuel vaporizes. As a fuel tank cools down, a vacuum is generated therein as fuel vapors condense to liquid fuel. Vacuum generation is monitored and leaks identified based on expected vacuum development or expected rates of vacuum development.
In order to preserve battery charge, a typical EONV test is subject to a time limit. A failure to reach a pressure or vacuum threshold before the end of the time limit may result in degradation being indicated, even if the fuel system is intact. The pressure rise portion of the test may execute until the fuel tank pressure curve reaches a zero-slope. If the pressure rise has a relatively low rate of constant increase (e.g., due to cool ambient conditions counteracting the pressure increase), and a significant amount of the time limit elapses prior to a zero-slope moment, the subsequent vacuum test may fail based on the limited amount of time remaining, regardless of the state of the fuel system.
Further, the entry conditions and thresholds for a typical EONV test are based on an inferred total amount of heat rejected into the fuel tank during the prior drive cycle. The inferred amount of heat may be based on engine run-time, integrated mass air flow, etc. However, the timing of heat energy transfer to the fuel tank significantly effects the fuel tank temperature at the initiation of the EONV test. A period of high-speed driving followed by a period of idling would indicate a high total amount of heat rejected, but much of the heat would dissipate from the tank during the idling period.
An alternative to relying on inferred sufficient heat rejection to determine suitable conditions and thresholds for entry into a typical EONV test is to instead actively pressurize the fuel system via an external pressure source. Toward this end, US Patent Application No. 2015/0090006 A1 teaches conducting leak detection in an evaporative emission systems control system by using a pump configured to both pressurize and evacuate the fuel system. However, the inventors herein have recognized potential issues with such a method. For example, the use of an external pump introduces additional costs, occupies additional space in the vehicle, and includes the potential for malfunction.
Thus, the inventors herein have developed systems and methods to at least partially address the above issues. In one example, a method is provided comprising, at engine shut-down, applying pressure stored in a coolant system degas bottle to a fuel system. In this way, pressure build-up in a degas bottle is advantageously utilized instead of being released to the atmosphere, thus obviating the need for an onboard pump in a case where an external pressure source is desired.
In one example, the pressure from the degas bottle may be applied to the fuel system during a leak detection test in order to pressurize the fuel system. Once the fuel system is pressurized, pressure decay may be monitored, and if the pressure decay does not meet a condition relative to a threshold (e.g., if the pressure decay rate is faster than expected), a leak in the fuel system may be indicated.
As one example, pressure stored in the coolant system degas bottle may be applied to the fuel system only under conditions wherein sufficient heat rejection is not inferred from the prior drive cycle. In this way, an evaporative emissions system leak test may be enabled under conditions where the leak test may otherwise not be executed, thus increasing opportunities for evaporative emissions system leak detection and correspondingly reducing bleed emissions.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.